This disclosure relates to a method for the detection of the presence of high atomic number elements. This disclosure also relates to a method for detecting the atomic number of hitherto unknown elements in an article.
The modern global economy relies heavily on intermodal shipping containers for rapid, efficient transport of ocean-going cargo. However, the possibility of concealing weapons of mass destruction (WMDs) and radiological dispersal devices (RDDs) in these containers represents a potential interruption to the free flow of commerce. Materials of concern such as uranium and plutonium that can be used to make nuclear weapons are characterized by having a high atomic number (high-Z). Similarly, radiological sources can be shielded employing high-Z materials to prevent these from being detected. Current x-ray inspection systems are not capable of detecting such materials and distinguishing them from common materials with a low false alarm rate.
Currently, when a radiographic image of an article is made, the intensity of various elements seen in the radiographic image represents the product of the materials linear x-ray attenuation coefficients and the path lengths at each pixel. A given product can be produced by different combinations of attenuation coefficients and path lengths. For example, a section of an article that has a thickness ‘x’ and is made from a material that has a low atomic number can produce the same intensity in a radiographic image as another section of the same article that has a thickness less than x but is made from a material that has a higher atomic number.
FIG. 1 is a graphical representation of the radiographic imaging of a multi-layered article produced with a high energy X-ray source called a linac for which the X-ray energies are determined by the voltage setting of the linac. A convenient term for indicating the total attenuation in a given object or objects is called the p-value, defined as shown in Equation (I) below:
                    P        =                  -                      log            ⁡                          (                                                I                  object                                                  I                  air                                            )                                                          (        I        )            
where Iobject is the measured energy with the article at a given pixel or a given collection of pixels, while Iair is the measured energy of the radiation without the article measured at the at the same pixel or given collection of pixels.
In FIG. 1, the ratio of the p-values at two different X-ray source voltage settings, 6 MV and 91 MV, is shown as a function of the p-value for the lower energy setting. As can be seen in the FIG. 1, when a sample comprising a 5 centimeter thick uranium slab wrapped in paper having a thickness of 2 meters is radiographically imaged using an xray source, the p-value is similar to the p-value of a sample comprising a 5 centimeter thick uranium slab and a 10 centimeter thick iron slab wrapped in paper having a thickness of 1 meter.
Thus, the simple radiographic imaging of the p-values for composite samples comprising multiple layers of different materials does not produce results that can facilitate a distinction or identification of different types of materials. It is therefore desirable to have a radiography device and a method that can distinguish between materials having different atomic numbers, when these materials are present individually or when they are in the presence of other materials.